Evolution describes the process by which heritable characteristics of biological populations change over successive generations. This fundamental concept in biology explains the diversity of life on Earth. Evolution is not a single event but rather an ongoing process driven by several distinct mechanisms that alter the genetic makeup of populations over time.
Natural Selection
Natural selection is a primary mechanism driving evolutionary change, acting on variations within a population. Individuals within any given species exhibit differences in their traits, such as size, coloration, or metabolic efficiency. These variations arise from genetic differences and can be passed down from parents to offspring.
Organisms often produce more offspring than their environment can support, leading to competition for resources. Within this competition, individuals possessing advantageous traits are more likely to survive and reproduce successfully than those with less favorable traits. For instance, a chameleon with matching skin coloration has a greater chance of avoiding predators, increasing its likelihood of survival and reproduction.
Over time, these favorable traits become more common in the population. This process leads to populations becoming adapted to their environments. An example is antibiotic resistance in bacteria; those with mutations allowing survival reproduce, leading to resistant strains. Natural selection is not a random process; it systematically favors traits that enhance survival and reproduction within a particular environment.
Genetic Drift
Genetic drift is another mechanism of evolution, distinct from natural selection, as it involves random changes in the frequency of alleles within a population. This process is particularly pronounced in small populations, where chance events can have a significant impact on genetic composition. Unlike natural selection, which favors beneficial traits, genetic drift operates purely by chance, meaning certain alleles may become more or less common due to random sampling.
One form of genetic drift is the bottleneck effect, which occurs when a population experiences a drastic reduction in size due to an environmental event. The surviving individuals represent only a random subset of the original population’s genetic diversity. For example, if a large insect population is decimated by a flood, the few survivors might by chance possess a disproportionately high frequency of a particular allele, even if it provided no survival advantage. The reduced genetic diversity can persist for many generations, making the population more vulnerable to future environmental changes.
Another manifestation is the founder effect, which happens when a small group of individuals migrates to a new area and establishes a new population. These “founder” individuals carry only a fraction of the genetic variation present in their original population. For instance, if a small number of birds from a mainland population colonize a new island, the island population’s genetic makeup will reflect only the alleles present in those initial colonizers. This can lead to a new population with unique allele frequencies, some of which may be rare in the source population, due to the random chance of initial colonization.
Gene Flow
Gene flow, also known as gene migration, involves the transfer of genetic material from one population to another. This process occurs when individuals move between populations and interbreed, introducing new alleles or altering existing frequencies. For example, if pollen from a plant in one meadow is carried by wind to fertilize a plant in an adjacent meadow, genetic material has flowed between the two plant populations.
The movement of individuals or gametes can introduce genetic variation into a population. This influx of new genetic information can increase genetic diversity, providing new raw material for other evolutionary mechanisms. Gene flow also tends to reduce genetic differences between populations by making their gene pools more similar over time. If two populations are continuously exchanging individuals, their genetic compositions will become increasingly homogenized, preventing them from diverging into distinct species.
Mutation
Mutation serves as the ultimate source of new genetic variation. A mutation is a random change in the DNA sequence of an organism. These alterations can range from a single nucleotide substitution to larger chromosomal rearrangements.
While many mutations are neutral, having no effect on survival or reproduction, others can be harmful. However, a small proportion of mutations can be beneficial, providing a new trait that enhances survival or reproduction. For example, a mutation might confer resistance to a disease or enable an organism to utilize a new food source. These beneficial mutations, though rare, are the raw material upon which natural selection and genetic drift can act.